Ethanol is readily produced by fermentation processes, which yield dilute aqueous ethanol mixtures. However, the dehydration of ethanol from its aqueous mixtures by traditional distillation methods requires large amounts of heat energy. Beverage grade ethanol is usually produced as an azeotrope containing 5 percent water by weight. For use in motor fuels, especially, gasohol, the ethanol must be substantially anhydrous.
Various methods for producing anhydrous ethanol, suitable for use in motor fuels are reviewed in Hartline, "Lowering the Cost of Alcohol", Science, Vol. 206, 41-42 (1979). Hartline describes only one adsorption process; using zeolite molecular sieves to selectively remove water from aqueous ethanol.
Oulman et al., U.S. Pat. No. 4,277,635 describe the use of a crystalline silica polymorph (silicalite) for the adsorption of ethanol from an aqueous ethanol mixture followed by recovery of the adsorbed, dehydrated ethanol by passing carbon dioxide gas through the silicalite bed.
Fornoff, U.S. Pat. No. 4,273,621 describes a gas phase distillation dehydration process using crystalline zeolite molecular sieves, and a carbon dioxide gas stream as a drying aid. This patent teaches that zeolite sieves having a pore diameter of three Angstroms are useful, because other adsorbents such as molecular sieves, carbon, alumina and silica would in addition to adsorbing water, coadsorb the ethanol and the carbon dioxide drying aid.
Zeolite sieves have one major drawback when used for the adsorption of water. They require a great deal of heat energy for desorption of the trapped water (i.e. regeneration).
None of the prior art discussed above, nor any of the references cited therein, suggest that carbon molecular sieves will be useful for dehydrating aqueous lower alkanol mixtures. It has been discovered that carbon molecular sieves having an average effective pore diameter of from 2.0 to 5.0 Angstroms are suitable for producing absolute ethanol, from an aqueous ethanol mixture having up to 60 percent water by weight. Moreover, carbon sieves, unlike zeolite sieves, are easily regenerated by methods such as those described herein.